1. Field of the Invention
The present invention relates to a method for making a barium titanate powder, in particular, by a solid-phase reaction. The present invention also relates to a barium titanate powder prepared by the above method, a dielectric ceramic compact formed by sintering the barium titanate powder, and a monolithic ceramic capacitor including the dielectric ceramic compact.
2. Description of the Related Art
A dielectric ceramic is prepared by sintering a powder mixture mainly containing a barium titanate powder, and is used to form dielectric ceramic layers which are components of monolithic ceramic capacitors.
Such a barium titanate powder is required to be as homogeneous as possible. In other words, the composition thereof must be homogeneous. Although barium titanate powders prepared by hydrothermal synthesis or hydrolysis show high homogeneity, the powders are expensive. Thus, barium titanate powders have generally been prepared by a solid-phase reaction.
In the synthesis of barium titanate by the solid-phase reaction, powdered barium carbonate and powdered titanium oxide are mixed and the mixture is calcined. It is very important to disperse powdered barium carbonate and powdered titanium oxide as homogeneously as possible in order to produce a more homogeneous barium titanate powder by the solid-phase reaction.
In general, homogeneous dispersions of powdered barium carbonate and powdered titanium oxide are prepared by dispersion processes which follow mechanical pulverization such as a dispersion process using dispersing media. Since powdered barium carbonate is hardly pulverized, it is difficult to obtain fine barium carbonate particles. As a result, the mixture of the powdered barium carbonate and the powdered titanium oxide exhibits unsatisfactory dispersion. Thus, the homogeneity of the barium titanate powder prepared by the solid-phase reaction is inferior to that prepared by the hydrothermal synthesis or hydrolysis.
An object of the present invention is to provide a method for making a barium titanate powder having high homogeneity by a solid-phase reaction.
Another object of the present invention is to provide a barium titanate powder prepared by the above method.
Another object of the present invention is to provide a dielectric ceramic compact formed by sintering the barium titanate powder.
Another object of the present invention is to provide a monolithic ceramic capacitor including the above dielectric ceramic compact.
The present invention is directed to a method for making a barium titanate powder comprising the steps of compounding powdered barium carbonate and powdered titanium oxide, and calcining the mixture, wherein the powdered barium carbonate comprises needle particles having an aspect ratio of the length of the long axis to the length of the short axis of at least about 2, wherein the long axis and the short axis of each particle are determined by electron microscopy.
Since the synthetic reaction of barium titanate involves diffusion of barium carbonate into a titanium oxide matrix, a uniform dispersion of the powdered barium carbonate in the powdered titanium oxide is important to improve homogeneity of the barium titanate powder. Since the powdered barium carbonate needle particles used in the present invention is readily pulverized into fine particles, the dispersion of the powdered barium carbonate is improved. Thus, a homogeneous barium titanate powder is obtained by a solid-phase reaction.
Preferably, the aspect ratio of the needle particles of the powdered barium carbonate is as high as possible. In other words, the aspect ratio of the length of the long axis to the length of the short axis is preferably at least about 5.
Preferably, the powdered barium carbonate has a BET specific surface area of at least about 5 m2/g.
The present invention is also directed to a barium titanate powder prepared by the above method. The barium titanate powder has a BET specific surface area of at least about 2 m2/g. Furthermore, the barium titanate powder satisfies the relationships D10/D50xe2x89xa70.25 and D90/D50xe2x89xa62.5 in particle size distribution by a laser diffraction scattering method. Moreover, the variation in molar ratio of barium to titanium in the barium titanate powder is about 0.01 or less wherein the molar ratio is determined by a TEM-EDX method using 10 samples. Herein, D10, D50 and D90 represent average particle diameters at a 10% cumulative volume, a 50% cumulative volume and a 90% cumulative volume, respectively, of particles when counted from the smallest particle, according to the laser diffraction scattering method.
The present invention is also directed to a dielectric ceramic compact prepared by sintering a green ceramic compact comprising a powdered feedstock comprising the above barium titanate powder.
The present invention is also directed to a monolithic ceramic capacitor comprising a plurality of dielectric ceramic layers, each comprising the above dielectric ceramic compact, and a plurality of internal electrodes, each extending along the interface between the two adjoining dielectric ceramic layers so that the two adjacent internal electrodes separated by the dielectric ceramic layer therebetween form an electrostatic capacitor.